The long-range goal of this research is to understand the molecular basis of cell proliferation and how it is deranged in cancer cells. The focus of this grant is on molecular and biochemical aspects of cell cycle control at the G2/M transition in Xenopus oocytes and eggs. Extracts from metaphase II-arrested eggs can be induced to cycle between mitosis and DNA synthesis by addition of free calcium, which mimics the natural signal at fertilization. This cycling reflects periodic changes in cyclin synthesis and degradation and periodic activation of the Cdc25 phosphatase by phosphorylation. One specific aim of this grant is to identify the protein kinases that phosphorylate Cdc25 since they may be mitotic triggers. Evidence supports Cdc2/Cdk2 as able to phosphorylate Cdc25, forming a positive feedback loop, but a distinct kinase for Cdc25 phosphorylation has been identified and its characterization and cloning is proposed. Related work will determine the mechanism of oscillation of PP1 in the cell cycle, focusing on phosphorylation of the catalytic subunit which has already been cloned, and association with regulatory subunits. Affinity- chromatography on microcystin-Sepharose will be a key element in identification of such subunits. A final aim in the grant is to characterize the novel membrane steroid receptor in oocytes that triggers transit of the G2/M transition (oocyte maturation). Monoclonal antibodies against the photo-affinity-labeled receptor will be used to screen oocyte cDNA expression libraries for cloning the gene. Antibodies and recombinant receptor will be used to characterize receptor function and distribution in other cells and tissues. The work proposed in this grant should help elucidate the molecular control of the cell cycle at the G2/M transition in normal cells, providing a framework for comparison with the controls in cancer cells.